1. Field of the Invention
The present invention relates to apparatuses having mushroom structures. Such apparatuses can be used not only for a reflector which reflects a radio wave in a specific direction, but also for an antenna at the time of transmitting and receiving a radio wave, a filter which attenuates a specific frequency, etc.
2. Description of the Related Art
In mobile communications, when there is an obstacle such as a building in a path of a radio wave, a received level deteriorates. To this end, there is a technique in which a reflector is provided at an elevation as high as that of the building and in which a reflected wave is transmitted to where the radio wave is hard to reach. When the radio wave is reflected by the reflector, it becomes difficult for the reflector to direct the radio wave in a desired direction if an incident angle of the radio wave within a vertical plane is relative small (FIG. 1). This is because, in general, the incident angle and a reflection angle of the radio wave are equal. In order to deal with this problem, it is possible to slant the reflector such that it looks into the ground. In this way, the incident angle and the reflection angle may be made large relative to the reflector, making it possible to direct an incoming wave in a desired direction. However, it is undesirable from a viewpoint of safety to slant to the ground side a reflector which is provided at an elevation as high as that of the building which blocks the radio wave. From such a viewpoint, a reflector is desired which allows directing a reflected wave in a desired direction even when an incident angle of a radio wave is relatively small.
As such a reflector, there is a structure such that elements in the order of half a wavelength are periodically arranged. However, such a structure becomes significantly large. On the other hand, a reflect array in which a number of elements which are smaller than half a wavelength is attracting attention in recent years. One example of such a reflect array is a reflect array having mushroom structures.
With the reflect array which uses the mushroom structures, an inductance L and a capacitance C in an equivalent circuit are adjusted to adjust a resonance frequency to control a reflection phase and control a direction in which a radio wave reflects. Regarding schemes of adjusting the resonance frequency, there exists a scheme which displaces a position of a via from a center of a patch (see Non-patent document 1), a scheme which changes a size of the patch (see Non-patent document 2), a scheme which changes a voltage using a varactor diode (see Non-patent document 3), etc.
Non-patent document 1: F. Yang and Y. Rahmat-Samii, “Polarization dependent electromagnetic band gap (PDEBG) structures: Design and applications,” Microwave Opt. Technol. Lett., Vol. 41, No. 6, pp. 439-444, June 2004
Non-patent document 2: K. Chang, J. Ahn, and Y. J. Yoon, “Artificial surface having frequency dependent reflection angle,” ISAP 2008
Non-patent document 3: D. Sievenpiper, J. H. Schaffner, H. J. Song, R. Y. Loo, and G. Tangonan, “Two-dimensional beam steering using an electrically tunable impedance surface,” IEEE Trans. Antennas Propagat., Vol. 51, No. 10, pp. 2713-2722, October 2003
In order to realize a reflect array which directs a radio wave in a desired direction using a large number of elements, elements which provide a predetermined reflection phase need to be aligned. Ideally, for a predetermined range of some structural parameters such as a patch size, it is desirable that the reflection phase changes in the whole range (two π radian=360 degrees) from −π radian to +π radian.
However, there is a problem that no matter which of the above schemes is used a range of reflection phase in a given frequency does not cover a wide range.